Process for pickling and passivating enclosed structures



y 1964 HERBERT MANFRED FREUD 3,132,975

SURNAMED JEAN FRASCH PROCESS FOR PICKLING AND PASSIVATING ENCLOSEDSTRUCTURES Filed May 31, 1960 PlC'KLlNG- i' pifikl l SOLD-1.01!introduced at bottom to displace air while filling the hollow structurel introduced at top to displace the pickling solution R i G while thehollow structure remains Filled with liquid i/complezmg Salts in watersolution introduced at the a top to displace rinsing COMPLEXlNG waterwhile the hollow structure remains filled with liquid I /Waferintroduced at top to displace the complaxing salts while hollow struc-RINSIN G ture remains filled with liquid Pa ssivatin agents In 501,111D". introduced PASSlVATl M6 at top to dis lace the water while hollowstructure remains Filled with liquid DRYING m DESSICATING 0xygen GasFILLING United States Patent ()fiice 3,132,975 PROCESS FOR PICKLWG ANDPASIVATENG ENCLOSED STRUCTURES Herbert Manfred Freud, sin-named JeanFrasch, Nanterre,

France, assignor to Society Framalite, Paris, France, a

corporation of France Filed May 31, 19st), Ser. No. 32,551 Claimspriority, application France June 4, 1959 8 Claims. (Cl. Mil-6.14)

This invention is addressed to pickling and passivating surfaces offerrous metals which are not exposed for treatment in a conventionalmanner, and it relates more particularly to a new and improved processfor pickling and passivating concealed surfaces of iron base alloys andferrous metals such as the internal surfaces of enclosed structures ofsubstantial dimension or capacity, as represented by heat exchangers,boilers, gas generators, liquid storage tanks, and other process orstorage equipment.

Numerous processes have been developed and used commercially for thepickling and passivating of surfaces of steel, iron, and iron basealloys which are readily accessible for treatment and for observation.Such exposed surfaces are available for cleaning either by mechanicalmeans, such as sandblasting, shot peening and the like, or by chemicalmeans, such as solution of the oxides in pickling acids and the like.When use is made of pickling solutions, application for removal of theoxides from the surfaces can be made by brush-coating or byspraycoating, or the surfaces can be wet by the pickling liquids uponimmersion of the article in the pickling bath.

Removal of the oxides by the pickling acids can be followed immediatelyby washing with aqueous medium for the removal of the pickling acidsfrom the cleaned surfaces of the metal. Rinsing to remove the picklingsolutions can be followed almost immediately either by passivating or bydrying to avoid reoxidation of the treated ferrous surface.

In the event that the pickling acids are allowed to remain on theferrous surface, because of inadequate rinsing, it

, has been found that the residual acids remaining on the surface arecapable of reacting even after drying, to cause undesirable corrosion orrusting. In the event that drying to remove moisture after pickling isincomplete or too slow, it has been found that the moisture present onthe surface will begin to react almost immediately in combination withthe oxygen in the air, to cause undesirable rust formation.

Such rinsing, passivating and drying operations can be easily andquickly carried out when concerned with metallic surfaces measurable ina few square meters or even as many as 10 square meters. The problembecomes quite different when one has to process surfaces of severalhundreds or thousands of square meters, and especially when suchsurfaces are not readily accessible, such as when they form the internalsurfaces of enclosures which may have a capacity of thousands of cubicmeters and when such concealed and inaccessible surfaces have to besubstantially completely pickled and passivated.

By way of example, a heat exchanger for use in nuclear generation ofelectrical power, wherein such heat exchanger may consist of 24 ferrules30 meters high, having a volume of 350 cubic meters and an internalsurface measurable as 30,000 square meters, all of which is combined ina compact structure connected with access piping of 300 mm. diameter.Because of the size, it is impractical to immerse such structure into apickling bath. In the alternative, the pickling solutions might beintroduced into the interior of the exchanger in amounts to fill theexchanger for reaction until the oxides (rust, scale, soldering flux andthe like) have been dissolved or stripped from the metal Walls.

3,132,975 Patented May 12, 1964 Removal of the acids by rinsing isimpractical with structures of such dimension because a number of hourswould be required for draining the pickling solution, and an equalamount of time would be required for replacement with water. Further,replacement with water would operate merely to dilute the acidsremaining on the surfaces, such that several washings would be requiredbefore adequate removal of the acids could be achieved. During each suchemptying and filling cycle, the treated surfaces, especially in theupper portions of the exchanger, would be exposed for substantialamounts of time to oxygen in the air while acid is still present on thesurface, whereby rusting would take place before protection by therinsing water.

On the other hand, rinsing by continuous flow of water through theexchanger would also require a number of hours (such as 8-10 hours)before the water rising from the outlet is free of acids, as evidencedby a neutral pH. Even then, ferrous ions present evenin lowconcentration in the rinsing water will flocculate as Fe(OH) when therinsing water is at a pH of about 5. These light flocs of Fe(OH) willdeposit on the metallic surfaces and, after drying, will give rise todust, thereby to interfere with the desirable operation of theapparatus.

It has been found that even when the apparatus has been perfectly rinsedand all acid products in the pickling bath have been completely removed,it is necessary to drain off the water before passivating the surfaces.Between the time that the water is drained off and the structure isfilled with the passivating solution, portions of the treated surfacewill be unprotected for as much as 4 or more hours, which is sufiicientto give rise to rusting.

Finally, when the surfaces have been passivated, it be comes necessaryto dry the surfaces quickly, preferably without the use of heat, becausewater vapor produced by such heat will not find easy escape and thuscause rusting of the apparatus. The passage of air through theappara-tus to accelerate drying is also objectionable since it willintroduce an important quantity of carbon dioxide which will be capableof forming carbonic acid upon solution in water which, in combinationwith the oxygen in the air, will react almost immediately to causerusting or corrosion.

It is an object of this invention to provide a process for pickling andpassivating large concealed surfaces on the interior of a housing, andit is a related object to provide a method and means for achievingeffective pickling and passivating of ferrous surfaces of substantialarea, even such area is concealed from View as the interior surface of ahousing.

FIG. 1 is a flow sheet illustrating the procedures of this invention.

The process embodying the features of this invention embodies thefollowing combination of steps:

(1) The picklingsolution is introduced into the interior of thestructure from an inlet in the bottom side until the structure is filledto overflowing. As the pickling solution rises through the structure,air is displaced from the structure in a manner to eliminate theformation of any air pockets, thereby to enable the liquid to come intofull contact with all of the surfaces. For this purpose,

' an overflow pipe is provided in the uppermost portion of acids, but itis preferred to make use of a solution of the type described in theFrench Patent No. 966,785, issued June 12, 1946, to Herbert ManfredFreud, also called Jean Frasch, comprising a mild pickling agent. Suchmild agents are preferred over strong acids such as hydrochloric acid orsulfuric acid, which tend to liberate hydrogen and induce embrittlementof the steel while also possibly creating gas pockets whereby thepickling solution is expelled from contact with the surface of themetal.

The time that the pickling solution is allowed to remain in reactivecontact with the ferrous surface depends upon the pickling solutionemployed and the amount of rust or scale to be removed. The end pointcan be determined by titration of the solution at predeterminedintervals to determine the increase in the amount of iron in thesolution. When the increase in iron dissolved over a 6-hour period isless than 0.2 gram per gallon, pickling can be considered to becomplete. With a pickling solution of the aforementioned patent, thiswill usually take about 24-48 hours.

(2) Upon completion of the pickling operation, the surfaces are rinsedwithout exposure of the pickled surfaces to air. For this purpose, therinsing water is circulated through the housing from the top to thebottom so that the housing will always remain filled with liquid,thereby to avoid the formation of voids or gas pockets. To maintain suchcondition, it is preferred to introduce the rinsing water at a rategreater than the rate of removal of liquid from the bottom of thehousing. This described balance is established by the use of an overflowpipe branched above the rinsing water inlet and operated either by meansof a trap or by an expansion vessel.

The incoming rinsing water displaces the pickling solution towards thebottom of the housing, with some dilution possibly taking place. Rinsingis thus continued until the liquid issuing from the bottom side of thehousing is practically neutral or at least above a pH of 6. While thiswill indicate substantially complete removal of acid, the presence ofFe(OH) in the wash water will be indicated by the appearance of a bluecoloration due to the formation of Prussian blue upon the addition ofpotassium ferrocyanide in the Prussian blue test.

(3) It is desirable to eifect removal of the Fe(OH) which otherwisewould adhere to the walls of the housing, and give rise to dusting, aspreviously described. For this purpose, the surface is treated with acompound capable of complexing reaction with the contained Fe(OH) toform a soluble compound which can be removed in subsequent rinses toeffect substantially complete cleaning of the surfaces. Such complexingcompounds as citrates, tartrates, malates, and so on, can be introducedin solution in aqueous medium in the latter stages of the rinsingoperation, or immediately thereafter as a separate rinse. To conserve onthe amount of complexing compound utilized, it is desirable to separatethe rinsing cycle from the complexing or solubilizing cycle so as toenable recycling of the complexing solution. The circulation of thecomplexing solution from the top to the bottom of the housing fordisplacement of rinsing water without the formation of voids or airpockets is continued until the Prussian blue test with potassiumferrocyanide is negative.

(4) The complexing cycle is followed by a rinsing cycle to eliminatecomplexing compounds from the system. This is again accomplished by thecirculation of clean Water through the housing, as previously described,in a manner to keep the housing filled with water until the pH of thewater issuing from the housing is the same as that of the waterintroduced.

(5) The surfaces are then passivated by introduction of a passivatingsolution into the housing to replace the rinse water, preferably byintroduction of the passivating solution through the inlet at the topand drainage of the water from the outlet at the bottom, always keepingthe housing filled with aqueous medium. Again, to conserve on the amountof material employed, it is desirable continuously to recirculate thepassivating solution through the housing until the reaction iscompleted. In most instances, this can be achieved in from l-24 hours.

For passivation, use can be made of any of the well known or commercialpassivating agents for iron, such as an oxidizing agent in solution, butit is preferred to make use of a dilute solution of an amine or amideacid bichromate. When passivation has been completed and the surfacesare stabilized against rapid rusting or corrosion, the liquids aredrained from the housing for the first time to empty the housing sincepickling.

(6) It will be found that the Walls of the housing will still be Wetwith large amounts of water present as a thin film of a few microns inthickness. It is desirable to effect rapid removal of the water fordrying the surfaces before rusting can again take place. In accordancewith the practice of this invention, the opening or openings at the topof the housing are sealed and a suction is drawn from the inlet at thebottom. This operates to draw down liquid. For most efficient use, it isdesirable to repeat the suction steps at frequent intervals to provide apulsating effect whereby as much as of the retained moisture can bedrained from the housing in a matter of a few hours.

(7) Suction drying is followed by complete drying by the circulation ofdry oxygen or ozonized oxygen from the bottom of the housing to the top,using a vacuum pump at the top to draw off oxygen to maintainsubatmospheric conditions within the housing. Thus the evacuation rateshould be in excess of the rate of introduction of the oxygen so as tomaintain vacuum conditions within the housing, thereby to permit theoxygen completely to penetrate the interior of the housing for contactwith the remaining moisture whereby the tnoisture evaporates to saturatethe oxygen for removal.

Desiccation with an oxygen stream is continued until the absence ofmoisture in the exhaust stream is indicated, as by means of absorbentpellets (such as blue silica gel, which turns red when wet).

(8) When the walls of the housing have been dried, the access openingsare closed and the apparatus is filled with the fluid with which it isadapted to be filled in use.

It will be understood that the description of the invention by referenceto a heat exchanger is made by way of illustration but not by way oflimitation, and that the described steps of pickling and passivatingmassive and concealed surfaces of ferrous metals may be equally appliedto boilers, cisterns, storage tanks, or other apparatus and structuresof ferrous metal.

It will be further understood that while best results will be secured bythe combination of successive steps described, simple modifications andalterations in the process and in the conditions thereof can be madewithout departing from the spirit of the invention, especially asdefined in the following claims.

I claim:

1. The process of pickling and passivating internal ferrous surfaces ofmassive hollow structures comprising introducing a pickling solutioninto the structure in an amount completely to fill the structure and ina manner to effect substantially complete removal of air thereby toprovide for complete wetting of the interior surfaces of the structurewith pickling solution, maintaining the pickling solution in contactwith the interior surfaces for a time sufficient to remove rust andcorrosion from said surfaces, introducing rinsing water into thestructure while simultaneously removing pickling solution, said rinsingwater being introduced at a rate at least as great as the rate ofremoval of the pickling solution whereby the interchange is effectedwithout exposure of the surfaces to air, continuing the introduction ofrinsing water until the water issuing from the structure issubstantially neutral, introducing a solution containing an agentcapable of forming a soluble complex with iron hydroxides remainingwithin the structure to remove the iron hydroxides from the structure bythe introduction of rinsing water for displacement of the complexingsolutions, introducing a passivating agent in solution into thestructure for replacement of the rinsing water, and maintain ing thepassivating solution in contact with the pickled surface untilpassivation is completed, each of the foregomg steps, with the exceptionof the first filling of the structure, being carried out withoutexposure of the surfaces to air, draining the passivating solution fromthe structure and then drying the pickled and passivated surfaces.

2. The process as claimed in claim 1 which includes continuouslyalternating a step of circulating the pickling solution through thestructure for dynamic reaction with a step of allowing the picklingsolution to remain static within the structure for static reaction.

3. The process as clahned in claim 1 which includes maintaining thepickling solution in contact with the surface until the amount ofadditional iron dissolved in the pickling solution corresponds to lessthan 0.2 gram per gallon over a 6-hour period.

4. The process as claimed in claim 1 in which the step of drying theinterior surfaces of the structure after the passivating solution hasbeen drained therefrom comprises creating a vacuum within the structurefor the displacement of moisture from the walls of the structure.

5. The process as claimed in claim 4 in which the suction is generatedat frequent intervals to provide a pulsating action for drawing moisturefrom the surface.

6. The process as claimed in claim 1 in which oxygen is circulatedthrough the structure after drainage of the passivating solution to takeup moisture remaining on the surface for removal from the structure.

7. The process as claimed in claim 6 which includes withdrawing thegases from the structure at a rate greater than the introduction ofoxygen to maintain vacuum conditions within the structure wherebygreater access between the oxygen and the wet surfaces is available.

8. The process as claimed in claim 1 in which the introduction of acomplexing solution is continued until the efiiuent from the structureis free from discoloration by the potassium ferrocyanide test.

References Cited in the file of this patent UNITED STATES PATENTS2,428,364 Frager Oct. 7, 1947 2,524,757 Brines et al. Oct. 10, 19502,773,623 Schuster et al Dec. 11, 1956 2,907,689 Kidder et a1. Oct. 6,1959 3,063,866 Mayer et a1 Nov. 13, 1962

1. THE PROCESS OF PICKLING AND PASSIVATING INTERNAL FERROUS SURFACES OFMASSIVE HOLLOW STRUCTURES COMPRISING INTRODUCING A PICKLING SOLUTIONINTO THE STRUCTURE IN AN AMOUNT COMPLETELY TO FILL THE STRUCTURE AND INA MANNER TO EFFECT SUBSTANTIALLY COMPLETE REMOVAL OF AIR THEREBY TOPROVIDE FOR COMPLETE WETTING OF THE INTERIOR SURFACES OF THE STRUCTUREWITH PICKLING SOLUTION, MAINTAINING THE PICKLING SOLUTION IN CONTACTWITH THE INTERIOR SURFACES FOR A TIME SUFFICIENT TO REMOVE RUST ANDCORROSION FROM SAID SURFACES, INTRODUCING RINSING WATER INTO THESTRUCTURE WHILE SIMULTANEOUSLY REMOVING PICKLING SOLUTION, SAID RINSINGWATER BEING INTRODUCED AT A RATE AT LEAST AS GREAT AS THE RATE OFREMOVAL OF THE PICKLING SOLUTION WHEREBY THE INTERCHANGE IS EFFECTEDWITHOUT EXPOSURE OF THE SURFACES TO AIR, CONTINUING THE INTRODUCTION OFRINSING WATER UNTIL THE WATER ISSUING FROM THE STRUCTURE ISSUBSTANTIALLY NEUTRAL, INTRODUCING A SOLUTION CONTAINING AN AGENTCAPABLE OF FORMING A SOLUBLE COMPLEX WITH IRON, HYDROXIDES REMAININGWITHIN THE STRUCTURE TO REMOVE THE IRON HYDROXIDES FROM THE STRUCTURE BYTHE INTRODUCTION OF RINSING WATER FOR DISPLACEMENT OF THE COMPLEXINGSOLUTIONS, INTRODUC ING A PASSIVATING AGENT IN SOLUTION INTO THESTRUCTURE FOR REPLACEMENT OF THE RINSING WATER, AND MAINTAINING THEPASSIVATING SOLUTION IN CONTACT WITH THE PICKLED SURFACE UNTILPASSIVATION IS COMPLETED, ECH OF THE FOREGOING STEPS, WITH THE EXCEPTIONOF THE FIRST FILLING OF THE STRUCTURE, BEING CARRIED OUT WITHOUTEXPOSURE OF THE SURFACES TO AIR, DRAINING THE PASSIVATING SOLUTION FROMTHE STRUCTURE AND THEN DRYING THE PICKLED AND PASSIVATED SURFACES.